This invention relates to an electric power processing device, and more particularly, to a low-mass bi-directional DC-AC power converter. The low-mass, bidirectional DC-AC power converter can be incorporated into, for example, an aircraft power conditioning unit that interfaces generation equipment with various load equipment utilizing independent voltages levels and frequencies.
Many industries can benefit from lightweight power conditioning systems that are also flexible in providing a variety of voltages of different magnitudes and frequencies. One such industry is the aviation industry where advances in aircraft design (both manned and unmanned) are necessitating new electric power system architectures. For example, emerging aircraft have 270 VDC electrical power equipment while still maintaining legacy 115 VAC/400 Hz or variable frequency equipment. The 115 VAC is generated by a power converter that uses the 270 VDC as its input.
FIG. 1 illustrates a related art power conditioning system for an aircraft. The power conditioning system includes generator 10, Generator Control Unit (GCU) 15, External Power Connection (EPC) DC ground cart interface 40, high-voltage battery 20, low-voltage battery 30, DC-DC converter 50 and inverter/transformer unit 60. A typical aircraft may have two power conditioning systems similar to that illustrated in FIG. 1.
Generator 10 typically includes a wound field synchronous motor (WFSM) 12 that is configured to be used as a generator. The output of generator 10 forms high-voltage DC bus 25 by rectifying the output of WFSM 12 using rectifier 11. GCU 15 controls the excitation voltage of WFSM 12 to maintain a desired DC voltage at the output of generator 10. High-voltage DC bus 25 supplies most of the electrical power for the aircraft, and high-voltage DC bus 25 may, for example, have a magnitude of 270 VDC.
Connected to high-voltage DC bus 25 is on-board high-voltage battery 20. During normal operation, the charge on high-voltage battery 20 is maintained by generator 10 via the high-voltage DC bus. A battery charger and disconnect switches (both features not shown) may be connected between high-voltage DC bus 25 and high-voltage battery 20. When generator 10 is not available or if the power from generator 10 is insufficient, the system may be configured such that high-voltage battery 20 provides power to high-voltage DC bus 25 to operate the equipment. In some modern, more electric aircraft, the high-voltage battery is not connected to the bus, but is separated by a contactor, which is closed only when the main generator fails.
The input power to DC-DC converter 50 is provided by high-voltage DC bus 25, and the output of DC-DC converter 50 forms low-voltage DC bus 35 that supplies control power to the system avionics. During normal operation, the charge on low-voltage battery 30 is maintained by DC-DC converter 50 via low-voltage DC bus 35. A battery charger and disconnect switches (both features not shown) may be connected between low-voltage DC bus 35 and low-voltage battery 30. If DC-DC converter 50 is not operational or if the power from DC-DC converter 50 is insufficient, the system may be configured such that low-voltage battery 30 will provide power to low-voltage DC bus 35. The magnitude of low-voltage DC bus 35 may be, for example, 28 VDC.
Inverter/transformer unit 60 is a DC-AC converter that provides power to legacy equipment that run on AC power. Inverter/transformer unit 60 gets its supply from high-voltage DC bus 25 and converts it to AC power at, for example, 115 volts, 400 Hz.
EPC DC ground cart 41 can be connected to the aircraft's high-voltage DC bus 25 through DC ground interface 40 when the aircraft is on the ground. EPC DC ground cart 41 powers the high-voltage DC equipment and also provides power to the 115 volt, 400 Hz equipment via inverter/transformer unit 60. Other systems may have an EPC AC ground card interface that connects directly to the legacy AC bus.
Inverter/transformer unit 60 represents a related art solution employing a DC bus inverter with an output isolation transformer. Inverter/transformer unit 60 may also be configured as a DC-DC converter with an isolation transformer or a DC-DC converter with bi-polar voltage and a direct DC-AC stage.
FIG. 2 illustrates an exemplary topology for inverter/transformer unit 60 shown in FIG. 1. The supply power from high-voltage DC bus 25 is fed to DC filter 61, which filters out any noise on the high-voltage DC supply power. In this example, the input DC supply is uni-polar, i.e., one leg of the DC supply is grounded to the chassis. Multi-phase inverter 62 receives the filtered DC voltage and produces a biased, poly-phase AC output that is sent to transformer 63. Transformer 63 provides isolation and converts the AC signal from multi-phase inverter 62 to a desired AC voltage. AC filter 64 receives the AC signal from transformer 63 and provides a filtered AC output. An AC signal at the output of transformer 63 may be sent to controller 65 as a feedback signal to adjust multi-phase inverter 62 if the output voltage from transformer 63 deviates from a desired voltage.
In the related art topology of FIG. 2, the output of AC filter 64 is a non-biased, isolated AC voltage supply. In addition, the topology is such that the power can flow in either direction (bi-directional). However, use of transformer 63 in an aircraft is not desirable because the transformer is heavy and awkward (approximately 70 lbs for a 6-10 kVA unit operating at 400 Hz).
FIG. 3A depicts a transformer-less topology, inverter 70, that can be substituted for the inverter/transformer unit 60 of FIG. 1. The negative DC rail of the DC voltage supply is grounded, i.e., at chassis potential. Therefore, although inverter 70 does not have a transformer, it produces an AC signal that is biased. That is, the output AC signal from AC filter 71 will not be centered at zero volts (see FIG. 3B). Multi-phase inverter 72 can be configured such that the “average” value of the output sine waves and the peak-to-peak values of the output sine waves can vary as shown by the solid curve and the dotted-line curve in FIG. 3B. However, because the DC bus negative is grounded, inverter 70 will produce output sine waves whose values are always positive.
An output sine wave that is always positive is a problem if the AC system is “expecting” a neutral referenced AC sine wave, i.e. a sine wave whose values are positive and negative (for example, the legacy AC system is typically 115 VAC/400 Hz). Therefore, in order to use the topology of FIG. 3A to provide a neutral referenced AC source, an isolation stage will be required between the uni-polar DC bus and the input side of inverter 70. This will increase the complexity, cost and weight of the system.
FIG. 4 depicts another related art topology, inverter 80, that can produce a non-biased, isolated AC output. In this topology, an isolated converter, DC-DC converter 86, is located between DC filter 81 and multi-phase inverter 82. DC-DC converter 86 has a high frequency transformer that provides the isolation for the system. The high frequency transformer is lower in weight (less than 10 lbs) than the transformer in FIG. 2. However, this topology also has drawbacks.
For example, DC-DC converter 86 must process the total power and derive a “new” isolated DC voltage that can be center tapped grounded to the chassis. In addition, DC-DC converter 86 does not provide bidirectional power flow. Therefore, an AC source, such as an EPC AC ground cart, cannot be used to generate the DC bus when the aircraft is grounded. To add bi-directional capability to the topology shown in FIG. 4, additional components and controls must be added to both sides of DC-DC converter 86, which increases cost, complexity and weight.
Accordingly, the related art power conditioning units are awkward and heavy (transformers), do not easily provide bi-directional power flow capability (i.e., without requiring additional components), and/or produce an output voltage supply that is not optimal for an aircraft. Therefore, it is desirable to have a transformer-less power conversion unit that produces a non-biased, balanced isolated AC voltage supply. Preferably, the power conversion unit can also produce positively biased AC outputs, negatively biased AC outputs and non-biased AC outputs.
Main engine start has traditionally been done using an air-turbine starter (ATS). The ATS uses compressed air generated from a compressor powered by the on-board auxiliary power unit (APU) (e.g., a small or dedicated device used to compress air such as an electric generator), an external ground cart or the aircraft's other engine (if there is more than one). In order to be more autonomous, however, the aviation industry is requiring that emerging aircraft start their main engines with less ground support. Accordingly, aircraft are being designed to start their engines electrically (i.e., without an ATS), which requires a strong DC source.
One option for providing the electrical start is to provide a ground cart that not only powers DC equipment as described earlier but is also sized to perform a main engine start by providing sufficient power to inverter 120 to run the WFSM 12 in engine-start mode or to assist the on-board battery in performing the main engine start. Another option is to use an EPC AC ground cart (not shown in FIG. 1) to provide supplemental power via an AC to DC inverter to help the on-board battery in performing the main engine start. A third option is to size the on-board battery to perform the main engine start without requiring supplemental power from an external ground cart. The third option allows the aircraft to be more autonomous than either of the other two options.
Unfortunately, all these options have their drawbacks. DC ground carts typically have a power limit of approximately 90 kVA or less, based on historical equipment and EPC connectors used on aircraft. High-power ground carts that are rated to perform main engine starts are not very common and are typically only found in the largest airports or military bases. Future aircraft will have to be flexible and have the ability to start their engines anywhere in the world. For the AC ground cart option, the additional circuitry needed to allow the EPC AC ground cart to assist in the main engine start will add additional weight and complexity to the related art power conditioning units. Although the third option of providing an engine start capable on-board battery will provide this flexibility, the weight and cost of the battery makes this option prohibitive.
In addition, related art power conditioning units are not designed to regulate the power going to the legacy AC bus during main engine start. Without regulation, main engine starts may create power fluctuations that cause power blackouts in the legacy AC system, which would necessitate longer aircraft commissioning times.
Therefore, along with having a transformer-less power conditioning unit that produces a non-biased, isolated AC voltage supply from the high-voltage DC bus, it is also desirable to have a power conditioning unit that will help enable electric main engine start by using commonly available EPC AC ground carts and low-power EPC DC ground carts to supplement the power from the on-board battery. Preferably, power to the legacy AC bus is regulated to minimize blackouts during main engine starts.